Also, what are the constraints? If it being portable wasn't an issue, I suggest a naquadah generator from the Stargate universe, or maybe a ZPM from the same.

A G-Diffusion system might do. Verses employing this technology: Star Fox, F-Zero.

Runs on phlebotinium according to what I know (Arwingpedia offers reasonable conjecture, however), so I doubt you'll need to explain much, making it fairly easy to use.

Can be used to both modify mass and produce energy. Commonly installed into fighter craft. Likely also found in vessels up to cruiser-weight, in accordingly large variations.

Known to be temperamental and only as reliable as plot and Legal allow.

A (small) plothole rescued from a badfic. I believe there's precedent for an Agent removing one from a fic and turning it over to Makes Things, back in the TOS era.

You might have to get in touch with Shepard for that.

A Mister Fusion from Back to the Future. It'll take anything as fuel, including running on your rubbish.